I. Field of the Invention
The present invention relates to wireless telecommunications. More particularly, the present invention relates to a novel and improved method for implementing a high-transmission-rate over-the-air interface.
II. Description of the Related Art
The IS-95 standard from the Telecommunications Industry Association (TIA), and its derivatives such as IS-95A and ANSI J-STD008 (referred to herein collectively as IS-95), define an over-the-air interface suitable for implementing a bandwidth-efficient digital cellular telephone system. To this end, IS-95 provides a method for establishing multiple radio frequency (RF) traffic channels, each having a data transmission rate of up to 14.4 kilobits per second. The traffic channels can be used for conducting voice telephony or for conducting digital data communications including small file transfer, electronic mail, and facsimile.
While 14.4 kilobits per second is adequate for these types of lower data rate applications, the increasing popularity of more data intensive applications such as worldwide web and video conferencing has created a demand for much higher transmission rates. To satisfy this new demand, the present invention is directed towards providing an over-the-air interface capable of higher transmission rates.
FIG. 1 illustrates a highly simplified digital cellular telephone system configured in a manner consistent with the use of IS-95. During operation, telephone calls and other communications are conducted by exchanging data between subscriber units 10 and base stations 12 using RF signals. The communications are further conducted from base stations 12 through base station controllers (BSC) 14 and mobile switching center (MSC) 16 to either public switch telephone network (PSTN) 18, or to another subscriber unit 10. BSC's 14 and MSC 16 typically provide mobility control, call processing, and call routing functionality.
In an IS-95 compliant system, the RF signals exchanged between subscriber units 10 and base stations 12 are processed in accordance with code division multiple access (CDMA) signal processing techniques. The use of CDMA signal processing techniques allows adjacent base stations 12 to use the same RF bandwidth which, when combined with the use of transmit power control, makes IS-95 more bandwidth efficient than other cellular telephone systems.
CDMA processing is considered a "spread spectrum" technology because the CDMA signal is spread over a wider amount of RF bandwidth than is generally used for non-spread spectrum systems. The spreading bandwidth for an IS-95 system is 1.2288 MHz. A CDMA-based digital wireless telecommunications system configured substantially in accordance with the use of IS-95 is described in U.S. Pat. No. 5,103,459 entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM," assigned to the assignee of the present invention and incorporated herein by reference.
It is anticipated that the demand for higher transmission rates will be greater for the forward link than for the reverse link because a typical user is expected to receive more data than he or she generates. The forward link signal is the RF signal transmitted from a base station 12 to one or more subscriber units 10. The reverse link signal is the RF signal transmitted from subscriber unit 10 to a base station 12.
FIG. 2 illustrates the signal processing associated with an IS-95 forward link traffic channel, which is a portion of the IS-95 forward link signal. The forward link traffic channel is used for the transmission of user data from a base station 12 to a particular subscriber unit 10. During normal operation, the base station 12 generates multiple forward link traffic channels, each of which is used for communication with a particular subscriber unit 10. Additionally, the base station 12 generates various control channels including a pilot channel, a sync channel, and a paging channel. The forward link signal is the sum of the traffic channels and control channels.
As shown in FIG. 2, user data is input at node 30 and processed in 20 millisecond (ms) blocks called frames. The amount of data in each frame may be one of four values with each lower value being approximately half of the next higher value. Also, two possible sets of frame sizes can be utilized, which are referred to as rate set one and rate set two.
For rate set two the amount of data contained in the largest, or "full-rate," frame corresponds to a transmission rate of 13.35 kilobits per second. For rate set one the amount of data contained in the full rate frame corresponds to a transmission rate of 8.6 kilobits per second. The smaller sized frames are referred to as half-rate, quarter-rate, and eighth-rate frames. The various frame rates are used to adjust for the changes in voice activity experienced during a normal conversation.
CRC generator 36 adds CRC data with the amount of CRC data generated dependent on the frame size and rate set. Tail byte generator 40 adds eight tail bits of known logic state to each frame to assist during the decoding process. For full-rate frames, the number of tail bits and CRC bits brings the transmission rate up to 9.6 and 14.4 kilobits per second for rate set one and rate set two.
The data from tail byte generator 40 is convolutionally encoded by encoder 42 to generate code symbols 44. Rate 1/2, constraint length (K) 9, encoding is performed.
Puncture 48 removes 2 of every 6 code symbols for rate set two frames, which effectively reduces the encoding performed to rate 2/3. Thus, at the output of puncture 48 code symbols are generated at 19.2 kilosymbols per second (ksps) for both rate set one and rate set two full-rate frames.
Block interleaver 50 performs block interleaving on each frame, and the interleaved code symbols are modulated with a Walsh channel code from Walsh code generator 54 generating sixty-four Walsh symbols for each code symbol. A particular Walsh channel code W.sub.i is selected from a set of sixty-four Walsh channel codes and typically used for the duration of an interface between a particular subscriber unit 10 and a base station 12.
The Walsh symbols are then duplicated, and one copy is modulated with an in-phase PN spreading code (PN.sub.I) from spreading code generator 52, and the other copy is modulated with a quadrature-phase PN spreading code (PN.sub.Q) from spreading code generator 53. The in-phase data is then low-pass filtered by LPF 58 and modulated with an in-phase sinusoidal carrier signal. Similarly, the quadrature-phase data is low-pass filtered by LPF 60 and modulated with a quadrature-phase sinusoidal carrier. The two modulated carrier signals are then summed to form signal s(t) and transmitted as the forward link signal.